A technology to recover an energy from a heat source for a medium-to-low temperature range lower than the temperature obtainable by burning a fuel such as heavy oil or petroleum, has been advanced, and power generation by ocean thermal energy conversion, geothermal binary power generation, waste heat recovery power generation, solar thermal power generation, heating by a heat pump, heat exchange by a heat pipe, etc. have been practically used or tested.
As a working medium to be used for such power generation, heat pump, etc., water, a hydrocarbon such as propane or butane, a fluorocarbon such as trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), chlorodifluoromethane (HCFC-22), trichlorotrifluoroethane (CFC-113) or dichlorotetrafluoroethane (CFC-114), or ammonia, has, for example, been known (Patent Documents 1 and 2).
Ammonia and hydrocarbons have some restrictions in their commercial use in view of safety problems such as toxicity, inflammability and corrosiveness or for such a reason that they are poor in the energy efficiency.
Many of fluorocarbons have attracted attention as prospective working media in view of their merits such as no substantial toxicity, non-inflammability, chemical stability, availability of various fluorocarbons having different standard boiling points, and evaluation and researches of fluorocarbons have been actively carried out.
However, among fluorocarbons, compounds having chlorine atoms have persistence in environment and are considered to be related to ozone depletion, and they are stepwisely being abolished. For example, chlorofluorocarbons (CFC) containing chlorine atoms and having all hydrogen atoms halogenated, have already been abolished completely in advanced countries such as Japan, U.S.A. and Europe. Further, with respect to hydrochlorofluorocarbons (HCFC) containing hydrogen atoms, their reduction towards total abolition by 2020 is being carried out in advanced countries. Further, perfluorocarbons (PFC) and hydrofluorocarbons (HFC) contain no chlorine atoms and are not influential over the ozone layer, but their influence over global warming has been pointed out, and they are regarded as compounds for global warming, emission of which to the atmosphere must be controlled.
Patent Document 1: Japanese Patent No. 3,580,811
Patent Document 2: JP-A-2006-335825